United States acid rain sources

Selection of pollution control methods is generally based on the need to control ambient air quaUty in order to achieve compliance with standards for critetia pollutants, or, in the case of nonregulated contaminants, to protect human health and vegetation. There are three elements to a pollution problem a source, a receptor affected by the pollutants, and the transport of pollutants from source to receptor. Modification or elimination of any one of these elements can change the nature of a pollution problem. For instance, tall stacks which disperse effluent modify the transport of pollutants and can thus reduce nearby SO2 deposition from sulfur-containing fossil fuel combustion. Although better dispersion aloft can solve a local problem, if done from numerous sources it can unfortunately cause a regional one, such as the acid rain now evident in the northeastern United States and Canada (see Atmospheric models). References 3—15 discuss atmospheric dilution as a control measure. The better approach, however, is to control emissions at the source. 

Even rain is not pure water. Reports from the U.S. Geological Survey show that it contains 2.3—4.6 ppm of soflds, or a yearly precipitation of 2.5—5 t/km. Recently (ca 1997), work conducted ia the United States and Europe has underscored the rather dangerous results of iacreased use of fossil fuels, where the SO and NO emissions that end up ia the rain lower its pH from 5.6 (slightly acidic) for uncontaminated rain, to acid rains. Such acid rain has serious effects on surface waters (1). About 40 x 10 t of SO and 25 x 10 t of NO were emitted ia the United States ia 1980. There are, however, encouragiag trends the 1970 Clean Air Act has led to a gradual reduction ia these emissions, bringing the SO emissions down from the previous levels cited by 10% by 1990, and the NO emissions down by 6%, with a consequent slight decrease ia rain acidity. A part of the Clean Air Act is also iatended to cap SO emissions from major poiat sources at 13.5 x 10 t (2). Between 1994 and 1995, total SO emissions ia the U.S. decreased remarkably by 13% and total NO emissions by 8%. 

Atmospheric emissions of sulphur dioxide are either measured or estimated at their source and are thus calculated on a provincial or state basis for both Canada and the United States (Figure 2). While much research and debate continues, computer-based simulation models can use this emission information to provide reasonable estimates of how sulphur dioxide and sulphate (the final oxidized form of sulphur dioxide) are transported, transformed, and deposited via atmospheric air masses to selected regions. Such "source-receptor" models are of varying complexity but all are evaluated on their ability to reproduce the measured pattern of sulphate deposition over a network of acid rain monitoring stations across United States and Canada. In a joint effort of the U.S. Environmental Protection Agency and the Canadian Atmospheric Environment Service, eleven linear-chemistry atmospheric models of sulphur deposition were evaluated using data from 1980. It was found that on an annual basis, all but three models were able to simulate the observed deposition patterns within the uncertainty limits of the observations (22). 

Acid rain is actually a catchall phrase for any kind of acidic precipitation, including snow, sleet, mist, and fog. Acid rain begins when water comes into contact with sulfur and nitrogen oxides in the atmosphere. These oxides can come from natural sources such as volcanic emissions or decaying plants. But there are man-made sources as well, such as power plant and automobile emissions. In the United States, two-thirds of all the sulfur dioxide and one-fourth of the nitrogen oxides in the atmosphere are produced by coal-burning power plants. 

Rain with a pH less than 5.6 is known as acid rain. Acid rain is caused by burning fossil fuels (especially coal) and by fertilizers used in intensive agriculture. These activities emit sulfur and nitrogen in gas compounds that are converted to sulfur oxides and nitrogen oxides. These in turn create sulfuric acid and nitric acid in rain. Acid rain may also be created from gases emitted by volcanoes and other natural sources. Acid rain harms fish and trees and triggers the release of metal ions from minerals into water that can harm people. The problem of acid rain in the United States has been addressed in recent decades by the use of scrubbers in coal burning power plants and catalytic converters in vehicles. 

It is informative at this stage to compare estimates of soil acidification attributable to the several natural and anthropogenic sources discussed in this section. Estimates are given in Table 5.1 of the most important sources of acidity in a forested soil of the southeastern United States. It is clear that acid rain is an important, but not necessarily dominant, contributor to the soil s gradual acidification. The analysis is different in areas where rainfall is even more acidic, and is affected by change in land use from forestry to agriculture. The annual contribution to soil acidification is seen in Table 5.1 to be a very small fraction of the total soil acidity at this particular... 

Acid rain controls in the United States proved to be not only a success but a bargain, replete with surprises. All the electrical power plants targeted by the 1990 CAAA met the Phase I deadlines for SO2 and NO, reductions in 1995. Indeed, there was initially substantial over-compliance. Affected emission sources also met the tighter Phase II standards in 2000. The utility... 

Air pollution occurs when the concentration of natural and/or man-made substances in the atmosphere becomes excessive and the air becomes toxic. Emissions from transportation, industry, and agriculture are man-made sources of air pollution. Primary pollutants are gases, liquids, and particulates dispersed into the atmosphere through either man-made or natural processes. In the United States, the primary pollutants are carbon monoxide, sulfur dioxide, nitrogen oxides, volatile organic compounds (VOCs), and particulate matter (soot, dust, etc.). Secondary pollutants are derived from primary pollutants that undergo a chemical reaction and become a different type of toxic material. In the United States, secondary pollutants are ozone, photochemical smog, and acid rain. 

Acid rain is a problem today due to the lai e amounts of acidic oxides being produced by human activities. When this precipitation falls on natural areas that cannot easily tolerate such acidity, serious environmental problems occur. The average annual pH of precipitation falling on much of the northeastern United States and northeastern Europe is between 4 and 4.5. Specific rainstorms in some areas where there are numerous sources of SO2 and NO have had pH values as low as 1.5. Complicating matters further is the fact that acid rain is an international problem—rain and snow don t observe borders (Figure 11.2). Many Canadian residents are offended by the government of the United States because some of the acid rain produced in the United States falls on Canadian cities and forests (Figure 11.3). 

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